Abstract The and functions, pathophysiology sheer enormity of the microbial biomass in the human intestinal tract, the co-evolution between humans the microbiota, and the established function of the gut microbes in regulating normal host physiologic are all consistent with the idea that alterations in gut microbial ecology play a role in the of several conditions including cancer.Radiotherapy (RT) is an established curative and palliative cancer treatment regimen, with approximately half of cancer patients with solid tumors receiving RT some time during their disease. Mounting evidence also suggests that high-dose hypofractionated radiation exerts potent immune modulatory effects, prompting immunological active tumor cell death inducing tumor associated antigen (TAA) cross priming with elicitation of anti-tumor CD8+ T cells, and abscopal effects. Although cancer whether be there have been groundbreaking responses to immunotherapy in certain malignancies such as lung and melanomas, so far, immunotherapy is effective only in a portion of patients. This raises the issue of there are additional important regulators of T cell function that are relevant to tumor control and could harnessed to enhance radiotherapy.In support of this hypothesis, we have generated preliminary results demonstrating that: a) Treatment with vancomycin caused a significant enhancement of the tumor inhibitory effect vancomycin enhanced the ability of RT to increase ovalbumin specific and IFN- ??producing CD8+ infiltrating T cells; c) gut gram+ depletion enhanced antigen presentation in the tumor draining lymph nodes and e) the observed synergy between vancomycin and RT in eliciting an anti-tumor immune response and inhibiting tumor growth was abrogated in IFN-? KO animals or by CD8+ cell depletion. Having established a direct link between the intestinal bacterial contents and radiotherapy, we propose the following three specific aims: Aim 1. To test whether the radiation enhancing effects of vancomycin are dependent on Gram+ bacteria, are transferable to another host and to identify the bacterial species responsible; Aim 2. To investigate the mechanism by which vancomycin treatment impacts both the priming and effector phases of RT-elicited antitumor immune responses; Aim 3. Perform a randomized pilot trial of antimicrobial therapy and stereotactic body radiation therapy in early-stage non-small cell lung cancer. Successful completion of these studies will firmly establish the microbiome as a target for therapeutic intervention in patients receiving radiotherapy and will identify markers of response in the clinical setting. of targeted radiation; b)